1. Field of the Invention
This invention relates to improvements in a catalytic converter for purification of exhaust gas from an automotive engine, and more particularly to such a catalytic converter to promote exhaust gas purification performance at low temperatures or at engine starting.
2. Description of the Prior Art
Catalytic converters having a honeycomb type monolithic catalyst have been proposed as disclosed in Japanese Utility Model Provisional Publication No. 57-44921 and extensively used in automotive vehicles. One of such a conventional catalytic converter will be briefly discussed with reference to FIGS. 7 and 8. The catalytic converter in FIG. 7 includes a single honeycomb type three-way catalyst 3 which is disposed inside a metallic casing 1 through a mat 2 formed of ceramic fiber. The casing 1 is constituted of upper and lower counterparts and has exhaust gas inlet and output openings 1A, 1B. The honeycomb type three-way catalyst 3 includes a honeycomb grid type ceramic substrate in which many axially extending cells are formed. Each cell is defined by ceramic walls. The walls of the substrate are coated with so-called wash coating which is formed of alumina and contains cerium, lanthanum and/or the like. Additionally, catalytic noble metals such as platinum (Pt), rhodium (Rh) and/or palladium (Pd) are carried by the wash coating to accomplish catalytic action for noxious components such as nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons contained in exhaust gas.
Now a temperature of about 300.degree. C. is necessary to activate such a catalyst so that the catalyst exhibits a sufficient exhaust gas purification performance. Accordingly, a considerable time is required to attain such a temperature particularly at engine start-up in which the catalyst is at a low temperature. Since exothermic reaction of exhaust gas occurs in the catalyst, temperature distribution within the catalyst is as shown in FIG. 8 in which temperature is higher in the central and outlet sections than in the inlet section of the catalyst. In general, the central and outlet sections of the catalyst are 50.degree. to 100.degree. C. higher than in the inlet section. As a result, it is required to raise the warm-up characteristics of the inlet side of the catalyst.
In order to raise the warm-up characteristics of the catalyst inlet section, it is considered to increase the amount of the catalytic metals, to reduce the amount of the wash coating, to reduce the amount of cerium in the wash coating, and/or to minimize the density of cells (i.e., the number of exhaust gas passages per a unit area).
However, in the conventional catalytic converter using a single catalyst, there becomes too much catalytic metal over a necessary amount in the central and outlet sides of the catalyst in the above-mentioned measure to increase the amount of the catalytic metals, thereby raising cost of the catalytic converter. According to the above-mentioned measures to reduce the amount of the wash coating and the like and/or cell density, there arise problems of deterioration of the catalyst at high temperatures and of lowering in exhaust gas purification performance.
In addition, other measures to improve the warm-up characteristics of the catalyst inlet section have been proposed as disclosed in Japanese Utility Model Publication No. 56-50716, Japanese Utility Model Publication No. 58-47207, Japanese Patent Provisional Publication No. 55-164715, Japanese Patent Provisional Publication No. 48-9975, and Japanese Patent Provisional Publication No. 49-41722.
A catalytic converter disclosed in Japanese Utility Model Publication No. 56-50716 includes two (upstream and downstream side) catalysts in which the upstream side catalyst is higher in the cell density than the downstream side catalyst. This leads to increasing the heat capacity of the upstream side catalyst thereby degrading the warm-up characteristics of the upstream side catalyst, which is apparent under the present state of the art technique in which a substrate or carrier of the catalyst is high in strength so that the thickness of walls of the cells can be made constant regardless of the cell density. Catalytic converters disclosed in the other publications also have two (upstream and downstream side) catalysts which differ from each other in the kind of catalyst and have different functions. Accordingly, complicated control is required to cause them to exhibit the respective functions. In case base metal is used for the catalytic metal of the catalyst, the catalyst is degraded in low temperature activity and purification performance for nitrogen oxides (NOx) and paraffin hydrocarbons at high temperatures, as compared with noble metals.